Multi-path communications are known in the wireless arts and are used to boost spectral efficiency. For example, the Third Generation Partnership Project (3GPP) Evolved Universal Terrestrial Radio Access system (E-UTRA, alternatively known as Long Term evolution of UTRA or LTE) supports both single-user (SU-) and multi-user (MU-) MIMO schemes. The performance of these MIMO schemes is highly dependent on the quality of channel state information (CSI) feedback obtained from the user equipment (UE). In LTE this CSI feedback comprises a precoding matrix indication (PMI), a channel quality indication (CQI) and a rank indication (RI). The PMIs are selected by the UE from a known codebook; one that is known in advance to both the network access node (eNB) and the UE. The codebook is typically specified in a published wireless protocol, and, where there is a choice of codebooks, the operative one can be made known to the UE via signalling. These codebooks have generally remained the same throughout earlier development of LTE: the codebooks for two and four transmit antennas have been specified already in Release 8 and the codebook for eight transmit antennas was specified in Release 10.
As part of development towards the Release 12 LTE specification, 3GPP is studying further enhancements to CSI feedback, in particular targeting deployments with four transmit antennas at the transmitter side. Specifically, it was agreed in the Radio Access Network Working Group 1 (RAN WG1) meeting referenced immediately below to investigate the so called double codebook (DCB) as a 3GPP Release 12 PMI enhancement for boosting MIMO performance in wireless communications involving 4-transmit (Tx) antennas. The DCB is standardised already in 3GPP Release 10 for 8-Tx antennas; see section 7.2.4 of 3GPP TS 36.213 v11.1.0 (2012-02). It was also proposed to accept this 8-TX codebook structure for 4-Tx antennas because it enables reuse of existing codeword (PMI) selection schemes at the UE-side implementation; see for example document R1-130258 by LG Electronics entitled “Considerations on CSI feedback structure for DL MIMO enhancement” [3GPP TSG RAN WG1 Meeting #72; St. Julian's, Malta; Jan. 28 to Feb. 1, 2013]. The exact codewords are however yet to be agreed.
Specification 3GPP TS 36.213 referred to above has a 4-Tx antenna codebook which was first standardised in Release 8. This 4-Tx codebook can be used as a DCB when an identity matrix is used as the wideband codeword W1.